This research will be done primarily at the laboratory of Dr. Jorge J. Casal, University of Buenos Aires as an extension of NIH grant RO 1GM52413, Initial Events in Photoreceptor Signaling. The coordination of different signals controlling the transcriptome is essential for proper development of multicellular organisms. The long-term aim is to investigate the mechanisms of signaling connectivity by using light signaling in Arabidopsis thaliana as a model system. The system combines developmental complexity and sophisticated genetics with the ability to reversibly activate different receptors. By using Affymetrix full-genome chips we will identify transcripts whose abundance is controlled synergistically by the photoreceptors, phytochrome B (PHYB) and cryptochrome 1 (CRY1), in detailed kinetic experiments. A combination of unsupervised and supervised multivariate analysis of the data will be used to investigate the main patterns of the transcriptome. This information will provide a dynamic picture of the architecture of the network. The genes will be prioritized according to earliness and significance of the synergism and their function will be investigated by using reverse genetics, taking advantage of the publicly available large population of T-DNA mutants where disrupted genes have been identified. Complementary to this analysis, we will identify signaling elements necessary for the synergistic interaction between PHYB and CRY1 using a novel genetic screen. Seeds randomly mutagenized by T-DNA and populations mutagenized by chemical agents will be used for the screens. The mutated genes will be cloned either by sequencing the areas surrounding the T-DNA insert or by chromosome walking. Mutants resulting from forward or reverse genetics will be characterized by their ability to integrate signals when exposed to light combining different wavelengths. The mechanism by which different photoreceptors regulate an overlapping, yet diverse, set of developmental responses has been a long-standing question. Light signaling may use shared components. Alternatively, the signaling pathways may converge on a similar set of target genes. The experiments proposed here are designed as an integrated approach to begin answering this important question.